1. Field of the Invention
The invention relates to phase locked loop circuits, and more particularly, to phase locked loop circuits capable of calibrating a gain of a voltage controlled oscillator.
2. Description of the Related Art
Many different types of integrated circuits and non-integrated circuits employ clock generating circuits such as phase locked loop circuits. Some examples of integrated circuits employing phase locked loop circuits include, but are not limited to, graphics processors, central processing units, measurement processors, microprocessors, and communication processors or any other suitable integrated circuits that employ clock generators. Manufacturing process variations, voltage supply variations and variations due to temperature changes, may cause a phase locked loop to go out of phase, thereby causing clocked data to be lost. Although phase locked loop circuits employ a feedback structure that can adjust, to some degree, phase variations, for wide frequency phase variation ranges, the feedback structure adjustment is unsatisfactory. For conventional voltage controlled oscillator calibration methods, gains of the phase locked loop circuits are detected by an offline calibration. For example, the phase locked loop circuits are required to switch to an open loop from a closed loop to perform gain detection steps. However, a phase locked loop may go out of phase when the phase locked loop is operated in an open loop during the offline calibration. Hence, in such a case, the gain of the phase locked loop is not detected and calibrated when a system is online. Accordingly, there is a need for an online gain calibration method for a phase locked loop circuit.